Relying how you chop it, the ray-finned fishes (Actinopterygii) are arguably probably the most numerous group of vertebrates within the trendy fauna. They’re the dominant vertebrates in all aquatic environments, they embody an infinite array of species, they usually have advanced a bewildering assemblage of morphologies. However regardless of their present pre-eminence, the early evolution of actinopterygians stays somewhat understudied. The earliest actinopterygians seem within the fossil document within the Late Silurian/Early Devonian however, till pretty not too long ago, the vast majority of Palaeozoic ray-finned fishes have usually been lumped right into a catch-all holding tank, the ‘Palaeonisciformes’. This was a obscure assemblage of fishes united by plesiomorphic options corresponding to ganoid scales (heavy, bony scales with an outer layer of enamel, additionally present in trendy gars and sturgeons), a single dorsal fin and a heterocercal tail (with the higher arm of the tail fin longer than the decrease). The important thing genus of the group, the Permian Palaeoniscum, had a fusiform (or torpedo-shaped) physique; at first look, it will not have seemed dissimilar to a contemporary herring. Nonetheless, it lacked the cell jaw construction of contemporary teleost fishes, with the maxilla and preopercular bones being fastened collectively. As such, it will have lacked the fashionable fish’s capability for suction feeding (Lauder 1980). Prey seize by Palaeoniscum would have been a easy smash-and-grab affair. Palaeoniscoid fishes remained a element of each marine and freshwater faunas till the tip of the Cretaceous earlier than being completely supplanted by trendy teleost radiations such because the ostariophysans and percomorphs.
The core idea of ‘Palaeonisciformes’ has united fishes with a fusiform physique form like Palaeoniscum; relying on the writer, extra divergent up to date fishes such because the deep-body platysomoids may be mixed in the identical order or handled individually. By trendy requirements, former ‘Palaeonisciformes’ most likely mix stem-actinopterygians, stem-chondrosteans, stem-holosteans and probably even stem-teleosts. As such, the time period Palaeonisciformes has tended to fall out of favour, although the much less formal ‘palaeoniscoid’ stays a helpful descriptor. Nonetheless, the precise phylogenetic place of many palaeoniscoid taxa stays unestablished. A part of this is because of an absence of observable element: although these heavy ganoid scales protect nicely, they successfully cowl up inside skeletal options. Many palaeoniscoids are preserved as compression fossils, successfully not rather more than intriguing silhouettes. Nonetheless, a part of the issue is straightforward neglect. Palaeoniscoids are usually not uncommon fossils; in some formations, they would be the dominant a part of the fauna by a big margin. They definitely deserve a better look.
Lauder, G. V., Jr. 1980. Evolution of the feeding mechanism in primitive actinopterygian fishes: a purposeful anatomical evaluation of Polypterus, Lepisosteus, and Amia. Journal of Morphology 163: 283–317.